1. Field of the Invention
The present invention relates to a system and a method for preventing scale defects on a finish rolling line of hot rolling equipment by descaling or cooling a surface of a material to be rolled (hot rolled steel plate) to suppress the formation of scale (oxide film).
2. Description of the Related Art
In hot finish rolling, oxide film may grow after scale removal at an entry side of a row of finishing mills. Depending on the thickness of the oxide film, scale defects occur in a hot rolled steel plate to decrease the yield of the product and deteriorate its surface quality. To suppress the growth of oxide film, it has been performed empirically to set the temperature of the steel plate surface, or to control the temperature of the steel plate surface at the entry side of the row of finishing mills. When the growth of oxide film was suppressed by any of these conventional methods, the hot rolled steel plate was overcooled, or occurrence of scale defects was not fully prevented.
FIG. 8 is a view showing another conventional method for preventing scale defects. In this drawing, a hot rolled steel plate 1, as a material to be rolled, passes, while being rolled, between a first stage rolling mill F1 and a seventh stage rolling mill F7 from an entry side to a delivery side (from left to right in the drawing). At a stage forward of the first stage rolling mill F1, a scale breaker FSB is placed for removing oxide film of the hot rolled steel plate 1 rough rolled by a roughing mill (not shown). High pressure water from a header 2 of the scale breaker FSB removes oxide film on the surface of the hot rolled steel plate 1. At an entry side of each of the second stage rolling mill F2 and the third stage rolling mill F3, descaling devices 12, 13 are placed. These descaling devices 12, 13 jet spray water when the thickness of oxide film on the surface of the steel plate is more than 10 .mu.m. After being so descaled, the steel plate is rolled.
However, when the descaling devices are arranged between the rolling mills as in FIG. 8 to perform descaling for oxide film more than 10 .mu.m thick, the thickness of oxide film at the entry side of the third stage rolling mill F3 may exceed 5 .mu.m as shown in FIG. 9. Finish rolling performed at an oxide film thickness of more than 5 .mu.m results in the occurrence of scale defects on the surface of the hot rolled steel plate 1, debasing the quality of the product. A thermometer 11 is provided at the entry side of the row of finishing mills so that the thickness of oxide film is predicted from the temperature of the steel plate detected, as well as the speed of the steel plate. Actually, the distance from the position of temperature detection to the descaling devices is so short that descaling control tends to be performed with some delay.